The invention relates to couplings for non-ferrous pipe (or tubing) of plastic or fiberglass, for example. More particularly, the invention relates to glass reinforced resin pipe having premolded or contact molded end portions (male and female threads) for joining the end portions together.
Resin pipe reinforced with glass filamentous material is well known to the art and possesses a number of advantages over metal pipe, e.g., inertness, low weight, corrosive resistance, and high strength. These properties make such pipes particularly suitable for use in the chemical processing industry and in the oil industry for use as line pipe, down hole tubing, etc.
Threaded end portions of glass reinforced resin pipes have been formed by a number of methods. One method employs the use of discrete coupling members (bonded to the pipes) providing threaded adjacent ends to the pipes to be joined. However, since the adjacent ends of such pipes are joined by an interposed connector, the possibility for leakage is doubled. In addition, the time and effort necessary for assembly is increased as well as the possibility for damage during assembly.
The use of threaded pipe end portions is a considerable improvement over the employment of discrete coupling members. However, the formation of threaded end portions simultaneously with the formation of the pipe by virtue of a threaded coupling-like member over the end portion of a rotatable mandrel, for example, is also subject to some disadvantages. For example, care must be taken to avoid the entrapment of air in the resin in the formation of the threads and to assure uniform and complete filling of the threads. Thus, an extra step is introduced into the formation of the pipe. The application of release agents to the threaded nipple end must also be carefully applied to insure a good release of the thread and further to prevent any damage to the threads when the finished pipe is removed from the mandrel.
Another method involves scribing grooves in a finished pipe to form threads therein. Although this is a workable arrangement, it is not without drawbacks. The scribing procedure results in broken reinforcement threads and exposed fiberglass strands in the pipe. However, a joint having scribed grooves on the mating male and female members provides the distinct benefit of enhanced tensile strength in the area of the coupling when compared to coupling elements bonded or otherwise molded to the fiberglass pipe. In other words, the scribed grooves of a male/female coupling provide a direct mechanical interlock between the substrates of the joined pipes. Such pipes possess increased tensile strength in the coupling portion which corresponds to the relatively greater interlaminar shear strength of a scribed thread compared to a molded thread.
What emerges from the evolvement to date of fiberglass pipe male/female type couplings is a number of advantages and disadvantages associated with purely scribed grooves in the pipe on the one hand and paste threads contact molded to the pipe laminate on the other hand.
Scribed threads are ground into the pipe laminate and necessarily involve excess resin material (for the amount ground away). The grinding and scribing processes applied may involve broken or cut threads or exposed fiberglass strands and the potential rejection of the thread after the entire pipe has been formed. However, the ground or scribed male and female thread joint provides the distinct advantage of a mechanical interlock tied integrally to the pipe laminate. The interlaminar shear strength of ground threads is recognized to be relatively greater than that of paste or molded threads whereby the corresponding tensile strength of the joint is enhanced.
In regard to paste or contact molded threads formed on the pipe laminate ends to be joined, certain advantages and disadvantages also exist. The primary disadvantage resides in the fact that a paste or molded thread does not provide mechanical interlock between the joined pipe laminates because only an adhesive bond exists between the paste threads and the pipe laminate. However, the paste or molded threads provide a more consistent thread profile with a corresponding relatively smooth finish, resulting in relatively lower engagement, and more significantly disengagement, torque requirements. Furthermore, the paste or molded thread has less of a tendency to powder during engagement than does the relatively rough surface of a thread scribed into the pipe laminate material. Also, a paste or molded thread includes a more random orientation of reinforcement materials when compared to the reinforcement strands or threads which may be cut in the pipe laminate during grinding of a scribed thread. In other words, a random orientation of reinforcement materials with respect to the shear plane of the scribed threads results in a relatively stronger joint.
The above considerations are important in considering the application and use of the joint. For example, in an oil field situation utilizing a string of fiberglass pipes or tubes, it is obviously important that sufficient interlaminar shear strength exist at a joint. Put another way, it is important that the overall tensile strength of a pipe string not be unduly limited by potential separation of any one joint during use of the string. The tensile strength of the joints is also of significance in a horizontal pipe application. For example, a horizontal pipe typically will have a hydrostatic pressure associated with it which develops an end load on a pipe string which in turn exerts a tensile force on the pipe joints in an axial direction. In short, the tensile or axial force considerations in both a vertical and horizontal pipe string are the same with respect to the present invention.